1. Field of the Invention
This invention generally relates to antenna duplexers having surface acoustic wave filters and electronic devices employing the antenna duplexers.
2. Description of the Related Art
In these years, mobile telephones and mobile information terminals have been widespread rapidly along with the advancements in mobile communication systems. Competitive races among manufactures are intense on downsizing and high performance thereof. A mobile telephone employs both analogue and digital networks, and the frequency bands are wide-ranging, for example, 800 MHz to 1 GHz or 1.5 GHz to 2.0 GHz. An antenna duplexer (or simply referred to as duplexer) having a surface acoustic wave (hereinafter referred to as SAW) filter has been proposed for use in a mobile communication device.
A mobile telephone employs dual mode or dual band so as to be more sophisticated and respond to the diversified communication systems under the development environment these years. The dual mode refers to the use of both analog and digital networks such as TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). The dual band refers to the use of both 800 MHz and 1.9 GHz frequency bands, or 900 MHz and 1.8 GHz or 1.5 GHz frequency bands. Several developments have been carried out to produce more advanced parts including a filter to be used in the mobile telephone.
Meanwhile, a mobile terminal is demanded to be smaller in size and lower in price. The sophisticated duplexer is, in most cases, a dielectric duplexer (composed of dielectric materials only), a combination of a dielectric material and a SAW device, or a SAW duplexer (composed of SAW devices only).
The dielectric duplexer is large in size, and it is hard to downsize and reduce the thickness of the mobile terminal, when it is mounted on the mobile terminal. Even if the duplexer including the SAW device is employed, dielectric material and makes it impossible to downsize and reduce the thickness of the whole duplexer. One SAW duplexer is a module type and includes the conventional SAW filters. The SAW filters and a matching circuit are mounted on the printed board. Another SAW duplexer is an integrated type and includes a multilayered ceramic package having SAW filter chips for signal transmission and reception mounted thereon.
The SAW duplexer has a volume of ⅓ to 1/15 that of the dielectric duplexer, and can be downsized and reduced the height by almost ½ to ⅓. With thus downsized SAW duplexer, the cost can be lowered to be almost same as that of the dielectric duplexer.
A description will now be given of a conventional duplexer. FIG. 1 is a structural block diagram of the conventional duplexer. FIG. 2 is a graph showing frequency characteristics of the conventional duplexer. The horizontal axis in FIG. 2 denotes frequency. The frequency becomes higher to the right. The vertical axis in FIG. 2 denotes passband intensity. The passband intensity becomes higher to the top. Referring to FIG. 1, a duplexer 100 includes an impedance matching circuit (or simply referred to as matching circuit) 11, two SAW filters 12 and 13, a common terminal 14, a transmission terminal 15, and a reception terminal 16.
The common terminal 14 is connected to an external circuit that transmits and receives an electric wave through an antenna. The transmission terminal 15 is connected to another external circuit for signal transmission and outputs a signal having a desired center frequency. The reception terminal 16 is connected to an external circuit for signal reception and inputs the signal having the desired center frequency. Terminals paired with the transmission terminal 15 and the reception terminal 16 are respectively grounded.
Generally, the impedance matching circuit 11 and the SAW filters 12 and 13 are included in the multilayered ceramic package. The SAW filters 12 and 13 have different center frequencies. The SAW filter 12 has a passband center frequency F1, and the SAW filter 13 has a passband center frequency F2. Here, F2 >F1. The SAW filter 12 is a filter for signal transmission, and the SAW filter 13 is a filter for signal reception. Therefore, the SAW filter 12 is referred to as a transmission filter 12, and the SAW filter 13 is referred to as a reception filter 13. For example, the duplexer of 1.9 GHz range has a difference of 100 MHz in frequency between F1 and F2.
The matching circuit 11 is provided not to degrade the filter characteristics of the filters 12 and 13. Z1 is a characteristic impedance in viewing the SAW filter 12 from the common terminal 14. Z2 is a characteristic impedance in viewing the SAW filter 13 from the common terminal 14. If a signal having the frequency F1 is input from the common terminal 14 due to the function of the matching circuit 11, the characteristic impedance of the common terminal 14 corresponds to the characteristic impedance Z1 of the SAW filter 12, the characteristic impedance Z2 of the SAW filter 13 is infinite, and a reflection coefficient thereof is 1. If a signal having the frequency F2 is input from the common terminal 14 due to the function of the matching circuit 11, the characteristic impedance of the common terminal 14 corresponds to the characteristic impedance Z2 of the SAW filter 13, the characteristic impedance Z1 of the SAW filter 12 is infinite and a reflection coefficient thereof is 1.
A description will now be given of the conventional technique for downsizing the duplexer. Japanese Patent Application Publication No. 10-126213 (hereinafter referred to as Document 1) proposes a configuration of the duplexer that employs the multilayered ceramic package. Document 1 also proposes that the matching line pattern is provided on two layers for matching the phases. Japanese Patent Application Publication No. 8-18393 (hereinafter referred to as Document 2) also proposes that the matching line pattern is provided on two layers. Japanese Patent Application Publication No. 10-75153 (hereinafter referred to as Document 3) proposes that multiple matching line patterns are provided on two layers. Japanese Patent Application Publication No. 2001-339273 (hereinafter referred to as Document 4) proposes that multiple matching line patterns are provided in a circumferential region of the chip.
Document 1 employs the structure of the multiple ceramic packages. However, the chip is connected with wires. This causes a drawback in downsizing. Additionally, the matching circuits are respectively required for the two SAW filters, when the duplexer employs the package with which the SAW filters are hermetically sealed.
Document 1 and Document 2 describe that the matching line patterns are provided on two layers. However, the matching line patterns are provided on the antenna terminal only. No consideration is given to the matching line on the transmission terminal or reception terminal. This is because the duplexers of Document 1 and Document 2 are designed for 800 MHz band, which is not very much affected from a parasite impedance as compared to the affect from 2 GHz band. There is a problem in that a mismatch of the filters cannot be adjusted in the high-frequency duplexer such as 2 GHz band especially.
Document 3 describes that the multiple matching line patterns are provided for phase matching. However, on the transmission terminal or reception terminal, no portion of the matching line pattern is provided on two layers. There is the same problem in that the mismatch of the filters cannot be adjusted in the high-frequency duplexer such as 2 GHz range especially.
Document 4 describes that multiple matching line patterns are provided in the circumferential region of the chip. There arises a problem in that the package size is large and it is hard to be downsized.